Static mechanical and hardness tests

A cast iron is always produced according to the design requirements provided by the client. Said requirements almost always have to do with the mechanical behaviour of the material, which must therefore be accurately determined and described, using so-called mechanical tests. The most common tests are those used to assess static mechanical behaviour and hardness.

Static mechanical tests are essentially tensile tests, in which yield (Rp0.2) and ultimate (Rm) tensile stress and elongation at break (A5%) are determined under uniaxial stress conditions. Hardness tests, on the other hand, determine the resistance of the material to penetration by a point, of different shapes, depending on the type of test carried out. Hardness tests are particularly useful because, using relatively simple equipment, they rapidly provide preliminary, but sufficiently reliable information on the mechanical behaviour of the cast irons.

Tensile testing

Hardness testing

Brinell hardness test

Vickers hardness test

Impact resistance tests

In the course of its service life, due to the design operating conditions or accidental events, a mechanical component may be subjected to impacts and shocks, against which suitable resistance must naturally be provided. It is naturally necessary to know the impact resistance characteristics of a metal alloy beforehand, precisely in order to prevent possible impact-induced failure or malfunctioning.

The impact resistance test serves precisely to determine the impact resistance of a given metal alloy. A test specimen (55 mm long, cross-section 10 mm by 10 mm), notched if necessary, is subjected to an instantaneous impact by means of a pendulum with a “knife”. The total energy absorbed during the impact is the impact resistance. If the instrumented impact resistance test is carried out, the instantaneous energy absorption curve is also determined, with the possibility to distinguish between the energy absorbed during initiation and during crack propagation.

Charpy Impact Test

Dynamic mechanical tests

Almost all mechanical components, in the course of their service life, are subjected to mechanical stresses of a cyclic nature (for example, tensile-compression or “oscillating” tensile stresses between different stress values). Over time, said stresses may cause the deterioration of the material’s mechanical performance and premature failure. Knowledge of the dynamic behaviour of metal alloys, which can be investigated by means of cyclic stress tests, commonly called fatigue tests, thus becomes essential. By way of the alternating tensile or tensile-compression stresses, the initiation and propagation of a crack can be studied at laboratory level, up to the material’s failure. By changing the stress conditions (axial, bending, multi-axial, cycle ratios) so-called Woehler curves are created, determining the fatigue limit of the material, i.e. the level of stress amplitude below which, theoretically, the material exhibits infinite cycle resistance.

Fatigue test

Tribology testing

Another significant type of stress to which mechanical components are subjected is that associated with contact (and thus sliding or abrasion) with other materials. Depending on the reciprocal characteristics of the component and the antagonist, wear phenomena can be induced (in one or the other, or both), with progressive damage (up to critical levels) of the surface of the materials. Knowledge of the behaviour and wear resistance of a material is therefore particularly useful in the design stage.

To assess wear resistance due to rubbing or abrasion by the material, the most common configuration is the so-called “pin-on-disc”: a pin (typically made of tool steel or ceramic material) subjected to a well-defined load is made to slide (in dry or lubricated conditions) on a disc made of the alloy to be characterised. The coefficient of friction during the test and the time-dependent consumption of the disc are determined. The test is mainly performed by way of comparison to compare the behaviour of different categories of materials.

Test pin-on-disk
Test pin-on-disk

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